BEIRUT, March 29 (Reuters) - President Bashar al-Assad said on Thursday Syria would spare no effort to ensure the success of international envoy Kofi Annan’s peace mission but warned it would not work without securing an end to foreign funding and arming of rebels opposing him.

Assad is under international pressure to call his troops and tanks back to their bases, a year into a popular revolt against his iron rule. Fighting between troops and rebels killed at least 38 mo re people on Thursday, 15 of them soldiers.

The state news agency SANA quoted Assad, in a letter to the leaders of the BRIC economic powers, which include his key ally Russia, as saying “countries which support the armed groups with money and weapons must be persuaded to stop this immediately”.

At the same time, Britain said it was doubling non-military aid to opponents of Assad and expanding its scope to equipment, possibly including secure telephones to help activists communicate more easily without fear of detection and attack.

The aid, worth $800,000, “includes agreement in principle for practical non-lethal support to them inside Syria,” Foreign Secretary William Hague said.

Arab leaders at a summit in Baghdad endorsed the peace plan floated by Annan, the special U.N. and Arab League envoy on Syria, and - betraying scepticism about Assad’s commitment -- called for it to be implemented “immediately and completely”.

The six-point plan envisages a ceasefire, possibly under U.N. monitoring, a withdrawal of heavy weapons and troops from population centres, humanitarian assistance, release of prisoners and free movement and access for journalists to Syria.  Arab League leaders quietly dropped an earlier demand that Assad give up the presidency. The Annan plan, endorsed by the U.N. Security Council, makes no such demand, unlike a previous blueprint for change that was vetoed by Russia and China.

“The solution for the crisis is still in the hands of the Syrians as a government and opposition,” Arab League Secretary-General Nabil Elaraby told the summit gathering in the Iraqi capital, the first there in 20 years.

Earlier, the League took a line closer to the hardline stance of Qatar and Saudi Arabia, calling for Assad to transfer powers to a deputy to allow for a unity government and early elections.

But a shift in the Arab position transpired after Russia and China blocked a Security Council endorsement of that proposal and the Arab League’s own observation mission in Syria unravelled from internal splits, failing to stem the bloodshed.

Syria’s opposition groups continue to demand that Assad must go, and have not agreed to any peace talks with his government.

Elaraby called for the Security Council to issue a binding resolution to “not only stipulate the necessity of stopping the violence, but also finding a suitable mechanism to cease fire.”

One idea Annan is pushing is a U.N. observer mission, diplomats say, and Elaraby said there had been suggestions that there should be “Arab participation”.

ASSAD‘S DEMANDS

“President Assad said that for his mission to succeed, Annan must focus on drying up the wells of support for terrorism pointed at Syria, especially from countries which have announced that they are arming and financing the terrorist groups in Syria,” SANA reported.

“In return for the formal commitment to the success of Annan’s mission it is necessary to obtain commitments from the other parties to halt the terrorist acts by the armed groups and to withdraw the weapons of these groups and call on them to stop their terrorist acts ...” Assad’s letter said.

In an apparent reference to Turkey, a former ally that now hosts the rebel Free Syrian Army, he singled out “neighbouring countries that harbour these groups and facilitate their terrorist operations”.

Assad said a national dialogue would start soon, including “all groups which work for the security and stability” of Syria.

U.N. Secretary General Ban Ki-moon kept up pressure on Assad, saying he must turn his stated acceptance of the peace plan into action, to shift his country off a “dangerous trajectory” with risks for the entire region.

Syria sits at a crossroads of Middle East conflict bordering Turkey, Jordan, Israel, Iraq and Lebanon. Its 23-million population comprises a mix of faiths, sects and ethnic groups whose tensions resonate in neighbouring states.

“It is essential that President Assad put those commitments into immediate effect. The world is waiting for commitments to be translated into action. The key here is implementation, there is no time to waste,” Ban told the Arab League summit.

OPPOSITION MEETS BACKERS

In Istanbul, Syrian opposition representatives met to try to settle deep internal disputes before the arrival of Western foreign ministers for a “Friends of Syria” conference on Sunday to map out where the year-old uprising is heading.

The chances of Western powers deciding to arm the insurgents at this point appeared to be very remote.

Reports from the opposition Syrian Observatory for Human Rights, which monitors the violence, said at least 23 people and 15 government soldiers were killed nationwide on Thursday - in army raids on villages, in a rebel ambush and in clashes.

SANA said two colonels were assassinated in a morning attack in Aleppo, Syria’s second city, while on their way to work. It said gunmen kidnapped Air Force General Mohammad Amr al Darbas in Damascus province.

The United Nations says Assad’s forces have killed 9,000 people. Damascus blames foreign-backed “terrorists” for the violence and says 3,000 soldiers and police have been killed.

Western powers have expressed doubt about Assad’s stated willingness to implement the peace plan. Russia has urged Western-backed opposition groups to match Damascus and endorse the proposals of Annan, a former U.N. secretary general.

Syria’s big-power backers, Russia and China, have turned up the heat on Assad by endorsing the Annan plan, with the unspoken implication that if he fails to act on it, they may be prepared to back action by the U.N. Security Council.

“In our experience in Iraq, we see that arming both sides will result in proxy wars regionally and internationally. This option will provide the grounds for foreign military intervention in Syria,” Iraqi Prime Minister Nuri al-Maliki told the Arab leaders.